The present invention generally relates to a liquid jet recording head, more particularly to a liquid jet recording head capable of recording a gradational image.
In a non-impact recording method, a noise generated at the time of recording is exceedingly small. In a so called ink jet recording method which is an example of the non-impact recording method, it is possible to record an image without providing a particular process where an image is fixed on a normal paper. This ink jet recording method is particularly useful so that various recording systems using this method have been proposed.
In the ink jet recording method, droplets of recording liquid, so called ink, is jetted so as to fly from a nozzle, and then the droplets are adhered to a recording member such as a recording sheet. Because of this, an image is formed on the recording member. The ink jet recording method is classified into various systems based on the method used to generate droplets of recording liquid and to control the flying direction of the droplets.
A first system is, for example, disclosed in U.S. Pat. No. 3,060,429. This system is called the "Tele Type system". In the first system, droplets are generated due to an electrostatic force, an electric field between deflecting electrodes is controlled in accordance with the recording signal so that flying droplets are selectively adhered to the recording member.
A second system is, for example, disclosed in U.S. Pat. Nos. 3,596,275, U.S. Pat. No. 3,298,030 and the like. This second system is called the "Sweet system". In this second system, droplets are generated by a vibrator such as a continuously vibrating piezo-electric vibrator, and then the each of droplets is charged in accordance with the recording signal. The charged droplets fly between the deflecting electrodes between which the constant electric field is formed so that each of the flying droplets is adhered to a position according to the image signal on the recording member, that is, the image is formed on the recording member.
A third system is disclosed in U.S. Pat. No. 3,436,153. This third system is called the "Hertz system". In this third system, the electric field is formed between the nozzle and a ring-shaped electrode, and droplets of the recording liquid are generated and atomized by the continuously vibrating vibrator. That is, the intensity of the electric field between the nozzle and the electrode is controlled in accordance with the recording signal so that the tomizing state of each of the droplets is controlled. Then the gradational image corresponding to the atomizing state of each of droplets is recorded on the recording member.
A fourth system is disclosed in U.S. Pat. No. 3,747,120. This fourth system is called the "Stemme system". The A principle of this fourth system essentially differs from each of the principles of the three systems described above. That is, in this fourth system, a piezo-electric vibrator is provided corresponding to each of the nozzles jetting droplets of recording liquid in the recording head, and then image signals are selectively supplied to the piezo-electric vibrators. Each of the piezo-electric vibrators converts the recording signal into a mechanical vibration, and droplets of the recording liquid are jetted so as to fly from each of the nozzles in accordance with the mechanical vibration of a corresponding piezo-electric vibrator.
In the first, the second and the third systems, the main energy for generating droplets of recording liquid is electrical energy and each of the droplets is deflected due to the controlling of the electric field. Therefor, in the first system, the structure of the recording head is simple, however it is necessary to supply a high voltage to the electrodes for generating droplets, and it is difficult to provide a recording head having a multinozzle structure so that this system is unsuitable for quickly recording an image.
In the second system, it is possible to provide the recording head having the multinozzle structure so that this system is suitable for quickly recording an image, however the structure of the recording head is complicated, and it is necessary to perform an advanced controlling operation for generating droplets. In addition, satellite dots which are positioned around the regular dot are formed in the image on the recording member with ease.
In the third system, droplets of the recording liquid are atomized so that it is possible to form an excellent gradational image, however it is difficult to control the atomizing state of each of the droplets, and images easily overlap with each other. In addition, it is difficult to provide the recording head having the multinozzle structure so that the third system is unsuitable for quickly recording an image.
In the fourth system, the structure of the recording head is simple, and only droplets corresponding to dots making up the image are jetted and fly from the nozzle (the on-demand system) so that it is unnecessary to draw back droplets of recording liquid which are unused for recording image. In the first, the second and the third systems, it is necessary to draw back droplets of recording liquid which are unnecessary for recording an image. In addition, it is unnecessary to use the conductive recording liquid for recording an image so that it is possible to select variouse type of recording liquids. However, it is difficult to make the recording head. It is also extremely difficult to miniaturize the piezo-electric vibrator having a required resonance frequency so that it is difficult to provide the recording head having the multinozzle structure. Droplets of the recording liquid are jetted and flown from the nozzle by a mechanical energy such as the mechanical vibration of the piezo-electric vibrator so that the fourth system is unsuitable for quickly recording an image.
An ink jet recording system in which the disadvantages of the first through the fourth systems described above are eliminated is proposed. This ink jet recording system is disclosed in Japanese Patent Publication No. 56-9429. In the disclosed ink jet recording system, ink in a liquid cavity is heated so that a bubble is generated and pressure in the ink suddenly increases. Then, due to the increasing of the pressure in the ink, a droplet is jetted from a narrow capillary nozzle.
Furthermore, an improved ink jet recording system is disclosed in Japanese Patent Publication No. 59-31943. In the improved ink jet recording system, electric heat conversion elements each having a heating portion and being capable of controling the amount of heat generated are provided. A signal having the gradational information is supplied to each of the electric heat conversion elements so that each of the heating portions heats the ink in accordance with the signal. As a result, the gradational image is recorded on a recording medium such as a recording sheet.
A description will now be given of the structure of one of the electric heat conversion elements described above. The structure of one of the electric heat conversion elements is, for example, shown in FIGS. 1 through 7.
In FIGS. 1 through 5, a heat reserve layer 72 and a heater layer 73 are stacked on a base 71.
A pair of electrodes 74 and 75 are formed on the heater layer 73. There is a gap .DELTA.l between the pair of electrodes 74 and 75. A protection layer 76 is formed so as to cover the heat layer 73 and the pair of electrodes 74 and 75. A set of layers positioned in the gap .DELTA.l forms a heater portion.
In the structure shown in FIG. 1, the thickness of the protection layer 76 in the heater portion (.DELTA.l) decreases in the direction going from an end (B) of the electrode 74 to an end (A) of the electrode 75. Therefore, the amount of heat supplied to the liquid through the surface of the heater portion (which is the surface of the protection layer 76) to the liquid for a predetermined time increases in the direction going from the end (B) of electrode 74 to the end (A) of the electrode 75. That is, in the amount of heat supplied through the surface of the heat portion to the liquid, the thermal gradient is generated.
In the structure shown in FIG. 2, the thickness of the heat reserve layer 72 in the heater portion (.DELTA.l) decreases in the direction going from the end (A) of the electrode 74 to the end (B) of the electrode 75. Therefore, the amount of the heat radiation from the heater layer 73 to the base 71 increases in the direction going from the end (A) of the electrode 74 to the end (B) of the electrode 75. As a result, the amount of heat supplied to the liquid for the predetermined time increases in the direction (B) to (A).
In the structure shown in FIG. 3, the thickness of the heater layer 73 in the heater portion (.DELTA.l) decreases in the direction going from the end (B) of the electrode 74 to the end (A) of the electrode 75. Therefore, the resistance of the heater layer 73 increases in the direction going from the end (B) of the electrode 74 to the end (A) of the electrode 75 so that the amount of heat generated by the heater layer 73 increases in the direction going from (B) to (A). As a result, the amount of heat supplied to the liquid for the predetermined time increases in the direction (B) to (A).
FIGS. 4 through 7 also show plan views of the structures of the electric heat conversion element disclosed in Japanese Patent Publication No. 59-31943. In each of the structures shown in FIGS. 4 through 7, an electrode 82 is connected to an end of a heater portion 81 and an electrode 83 is connected to another end of the heater portion 81.
In FIG. 4, the planar structure of the heater portion 81 is rectangular. An area where the electrode 82 and the heater portion 81 are connected with each other is narrower than an area where the electrode 83 and the heater portion 81 are connected with each other. In each of the examples shown in FIGS. 5 and 6, the heater portion 81 has a planar structure in which the width of the center of the heater portion 81 is narrower than each of the widths of both ends thereof. In an example shown in FIG. 6, the planar structure of the heater portion 81 is a trapezoid. Each of the edges of the heater portion which are non-parallel with each other is connected to one of the electrode 82 and 83.
In an example shown in FIG. 7, the heater portion 81 has a planar structure in which each of the widths of both ends of the heater portion 81 is narrower than the width of the center thereof. In each of the examples shown in FIGS. 4 through 7, the current density in the heater portion 81 decreases in the direction going from a position (A) to a position (B). Therefor, the level of the power supplied to the heater portion 81 is controlled so that the area where the bubble is generated by the heating function of the heater portion 81 is changed. As a result, the size of the bubble is controlled so that the size of the droplet jetted from the nozzle is controlled. Thus, it is possible to record the gradational image.
However, In the examples shown in FIGS. 1 through 3, it is very difficult to form the structure in which the thickness of the heat portion changes by the thin film forming process. Even if possible, the cost of production would be very high. In the device having the structure in which a pattern of the heater portion changes as shown in FIGS. 4 through 7, the pattern can be broken at the narrowest portion thereof with ease so that the durability of the heater portion is poor.
A gradational image recording system recording the gradational image is also disclosed in Japanese Laid-Open Patent Application No. 63-42872. In this recording system, it is difficult to produce the heater portion in the same manner as the examples described above. In addition, a gradational image recording system is disclosed in Japanese Patent Publication No. 62-46358, Japanese Patent Publication No. 62-46359, and Japanese Patent Publication No. 62-48585. In this type of recording system, a plurality of heater elements are provided on one liquid path or one nozzle so that the number of control electrodes connected to the plurality of heater elements increases and it is difficult to increase density of the nozzles. A recording system disclosed in Japanese Laid-Open Patent Application No. 59-124863 and Japanese Laid-Open Patent Application No. 59-124864 has a heater portion for jetting droplets and another heater portion and bubble generator for generating bubbles. Therefor, it is difficult to increase the density of nozzles. Furthermore, in another recording system disclosed in Japanese Laid-Open Patent Application No. 63-42869, the number of generated bubbles is controlled so that the amount of ink jetted from a nozzle is controlled. It is generally impossible to supply much power to the heater element in the bubble jet type recording system. Thus, in this type of recording system, durability is poor.
As has been described above, in the conventional liquid jet recording system, there are disadvantages from the point of view of the durability of the system and the increase in the density of the nozzles in the system and so on.